Simple range sensing systems which can quickly determine the range to one target or point are well known. These systems can be used in many applications, from determining a golfer's distance to the hole they are playing, to surveying applications to calculate the distance between specific landmarks or points.
These systems normally employ a laser range finding system where a laser beam is trained onto a selected target, and a receiver associated with the system calculates a distance or range to the target through the propagation delay of the laser beam.
However, these systems cannot necessarily be used easily where the range to multiple targets needs to be calculated simultaneously or in a very short period of time. For example, these types of “single target” range finders cannot necessarily be used effectively in machine vision applications which employ range information for all targets with a scene as inputs to object identification algorithms.
One attempt to provide a range finding system for such applications can employ a laser range finder substantially as described above, which has the laser beam scanned over the region of interest and all targets within such a region. However, this approach is not ideal as mechanical systems are required to physically move the laser source to provide the scanning motion required. Furthermore, significant computation or processing power is also required to calculate the range of a number of discreet targets using the propagation delay associated with multiple scans of the laser beam. Range information must be calculated separately for each scanning movement of the laser, which will require high performance computer processing power to ensure that the range information can be provided at an effective or useful speed.
A discussion on scanned laser systems above can be found in U.S. Pat. No. 5,638,164 and in D. Tu, “Range Image Acquisition for Machine Vision”, Optical Engineering, 37(9), pp 2531-5, 1998.
Another alternative range sensing system has also been developed to address these problems associated with scanned laser range finding systems, and is discussed in U.S. Pat. No. 6,100,517. This system employs a light or energy source and associated light or energy sensor both of which are pulsed on and off at the same frequency. Energy from the pulsed source is reflected from the targets within a particular area or region back towards the energy sensor, which again is enabled or activated in a pulse manner so that this reflected light will only be sensed when the sensor is activated.
The selective activation of the sensor is implemented through a shutter or gate placed between the sensor and any targets within a scene, and this shutter is open and closed at the correct pulsing frequency required. The frequency at which the source and sensor are pulsed is selected so that the amount of light reflected from distant targets is cut off after a set propagation delay, as opposed to light reflected from near targets which has a lower propagation delay. Light will be received from near objects for a longer period of time than light reflected or scattered from more distant objects. Therefore, more light will be received from near targets than distant targets, which gives a light intensity value or reading for a target which is proportional to its range from the sensor.
This type of system can be implemented with relatively low cost components and does not require the level of computational processing power which the scanned laser range finder discussed above requires. However, there are additional variables present in the operating environment of the system which can cause inaccuracies or errors in the resultant data obtained.
Changes in ambient light level (and therefore the amount of light received by the sensor) will provide an offset error in the results obtained. Furthermore, targets within a particular scene which have relatively high or low reflectance properties will also create errors in the output obtained, as the amount of light received by the sensor will vary not just with the range of the target from the sensor. Such systems that derive range values from intensity information are also ultimately limited in range resolution by the dynamic range of the sensor employed. If a low cost or low quality sensor, then the performance or accuracy of the range value derived in turn suffers.
An improved range sensing system which addressed any or all of the above problems would be of advantage. Specifically a range sensing system which could be implemented using relatively low cost componentry, which did not require a high degree of computational processing power and which also did not suffer from inaccuracies due to changing ambient light levels nor variable reflectance properties of targets would be of advantage.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.